There will be decades, if not centuries, between the establishment of a Mars colony and the point at which it is capable of surviving indefinitely with no supplies from Earth whatsoever. In the interim it will be simple for any global-scale military power--the same ones writing the laws--to threaten a Mars colony by blockading its shipments. Launch sites are relatively few and can be easily monitored, launch windows are extremely predictable, and spacecraft are extremely vulnerable to space-based weaponry. There would be no need to spend time attacking Mars when you can just cut them off and let them starve.